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The discovery of the natural radioactive decay of uranium in 1896 by Henry Becquerel, the
French physicist, opened new vistas in science. In 1905, the British physicist Lord Rutherford -after defining the structure of the atom -- made the first clear suggestion for using radioactivity as
a tool for measuring geologic time directly; shortly thereafter, in 1907, Professor B. B. Boltwood,
radiochemist of Yale University, published a list of geologic ages based on radioactivity. Although
Boltwood's ages have since been revised, they did show correctly that the duration of geologic
time would be measured in terms of hundreds-to-thousands of millions of years.
The next 40 years was a period of expanding research on the nature and behavior of atoms,
leading to the development of nuclear fission and fusion as energy sources. A byproduct of this
atomic research has been the development and continuing refinement of the various methods
and techniques used to measure the age of Earth materials. Precise dating has been
accomplished since 1950.
A chemical element consists of atoms with a specific number of protons in their nuclei but
different atomic weights owing to variations in the number of neutrons. Atoms of the same
element with differing atomic weights are called isotopes. Radioactive decay is a spontaneous
process in which an isotope (the parent) loses particles from its nucleus to form an isotope of a
new element (the daughter). The rate of decay is conveniently expressed in terms of an isotope's
half-life, or the time it takes for one-half of a particular radioactive isotope in a sample to decay.
Most radioactive isotopes have rapid rates of decay (that is, short half-lives) and lose their
radioactivity within a few days or years. Some isotopes, however, decay slowly, and several of
these are used as geologic clocks. The parent isotopes and corresponding daughter products
most commonly used to determine the ages of ancient rocks are listed below:
Parent Isotope
Stable Daughter Product
Currently Accepted Half-life
4.5 billion years
704 million years
14.0 billion years
48.8 billion years
1.25 billion years
106 billion years
The mathematical expression that relates radioactive decay to geologic time is called the age
equation and is:
t=1/delta ln(1 + D/P)
t is the age of a rock or mineral specimen,
D is the number of atoms of a daughter product today,
P is the number of atoms of the parent product today,
ln s the natural logarithm (logarithm to base e), and
delta is the appropriate decay constant.
(The decay constant for each parent isotope is related to its half-life, t 1/2, by the following
t 1/2 = ln2/delta
Dating rocks by these radioactive timekeepers is simple in theory, but the laboratory procedures
are complex. The numbers of parent and daughter isotopes in each specimen are determined by
various kinds of analytical methods. The principal difficulty lies in measuring precisely very small
amounts of isotopes.
Literally thousands of dated materials are now available for use to bracket the various episodes
in the history of the Earth within specific time zones. Many points on the time scale are being
revised, however, as the behavior of isotopes in the Earth's crust is more clearly understood.
Thus the graphic illustration of the geologic time scale, showing both relative time and radiometric
time, represents only the present state of knowledge. Certainly, revisions and modifications will
be forthcoming as research continues to improve our knowledge of Earth history.
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Antoine Henri Becquerel (1852-1908)
One day, French scientist Henry Becquerel, while experimenting, kept uranium salts near an unexposed photographic film by accident.To his surprise
he found that the film got exposed to some radiation which could not be seen. Later he discovered that the uranium salts were giving out this radiation.
In the following year, i e 1897, Marie Curie and her husband Pierre Curie found more elements like polonium and radium that gave out radiation and
they called this phenomenon radioactivity.
Madame Curie [1867-1934]
Pierre 1859-1906